why it is that on my XTR-09:
-when I set the slack to 35mm: less vibration, smoother throttle control & pick up

- when I set it to 45~50 mm: a lot of vibes, throttle less smooth and pick up from 40~45 mph in top is horrible???????

When I got the bike (new) I noticed the chain slack was around 30 mm, I changed it to around 50. Now 2000 miles later, I decided to make it tighter than the 40~50 spec in the handbook & I got the above result.